Trexx25 said:
This is shown and explained in I think Chapter 123/124. When Ishida is fighting Kurotsuchi, and is paralyzed by his technique, Ishida employs Ransoutengia to use the Reishi around him to work himself like a puppet, which requires having complete mastery over the spirit particles around you. Kurotsuchi says that in examining and dissecting like 2660 Quincy's, not one of them could use Ransoutengia, the most powerful Reishi manipulating Quincy technique. Kurotsuchi then says that he "understands" and calls Ishida a "genius". This leads one to believe that to become a great Quincy, you have to be extremely intelligent.
I don't think he meant a literal genius, just saying it as Uryu being a really skilled Quincy.